Tentative fix for not removing newly internal functions

Functions are not removed even when made internal by DXILFinalizeLinkage
The removal code is called from alwaysinliner and globalopt, which are
invoked too early to remove functions made internal by this pass.

This adds a check similar to that in alwaysinliner that removes
trivially dead functions after being marked internal. It refactors
that code a bit to make it simpler including reversing what is
stored in the work queue.

Not sure how to test this. To test all the interactions between
alwaysinliner, DXILfinalizelinkage and any other optimization passes,
it kinda needs to be end-to-end.

Fixes #106139

Author: pow2clk

clang/test/CodeGenHLSL/remove-internal-unused.hlsl

@@ -0,0 +1,17 @@
+// RUN: %clang_dxc -T cs_6_0 %s | Filecheck %s
+
+// Verify that internal linkage unused functions are removed
+
+RWBuffer<unsigned> buf;
+
+// CHECK-NOT: define{{.*}}donothing
+void donothing() {
+     buf[1] = 1; // never called, does nothing!
+}
+
+
+[numthreads(1,1,1)]
+[shader("compute")]
+void main() {
+     buf[0] = 0;// I'm doing something!!!     
+}

llvm/lib/Target/DirectX/DXILFinalizeLinkage.cpp

@@ -18,19 +18,20 @@
 using namespace llvm;
 
 static bool finalizeLinkage(Module &M) {
-  SmallPtrSet<Function *, 8> EntriesAndExports;
+  SmallPtrSet<Function *, 8> Funcs;
 
   // Find all entry points and export functions
   for (Function &EF : M.functions()) {
-    if (!EF.hasFnAttribute("hlsl.shader") && !EF.hasFnAttribute("hlsl.export"))
+    if (EF.hasFnAttribute("hlsl.shader") || EF.hasFnAttribute("hlsl.export"))
       continue;
-    EntriesAndExports.insert(&EF);
+    Funcs.insert(&EF);
   }
 
-  for (Function &F : M.functions()) {
-    if (F.getLinkage() == GlobalValue::ExternalLinkage &&
-        !EntriesAndExports.contains(&F)) {
-      F.setLinkage(GlobalValue::InternalLinkage);
+  for (Function *F : Funcs) {
+    if (F->getLinkage() == GlobalValue::ExternalLinkage) {
+      F->setLinkage(GlobalValue::InternalLinkage);
+      if (F->isDefTriviallyDead())
+	M.getFunctionList().erase(F);
     }
   }